Based on the oscillatory characteristics, especially the resonant frequency of the measuring tube, the density can be determined. Such measuring apparatuses have a cross-sensitivity to the viscosity of the medium. This increases with decreasing measuring tube diameter. On the other hand, straight measuring tubes with small diameters enable a density measurement with very small sample amounts. Additionally, measuring tubes with small diameters tend to have a higher eigenfrequency, so that an exact frequency measurement is easy to implement. Insofar, density measurements with oscillating measuring tubes are quite attractive, wherein, especially in the case of measuring tubes with small diameters, the cross-sensitivity to viscosity must be taken into consideration.
Kalotay et al. describe in U.S. Pat. No. 5,359,881 a viscosimeter, which has a Coriolis mass flow measuring device with integrated pressure difference sensor, which ascertains the pressure drop across the measuring tubes of the Coriolis mass flow measuring device. Based on the mass flow ascertained by means of the Coriolis mass flow measuring device and the density ascertained by means of the Coriolis mass flow measuring device, the volume flow is ascertained, based on which, taking into consideration also the pressure drop, the viscosity is determined. The volume flow, which is taken into consideration for viscosity determination, is based, however, exactly on the defective density measurement.